Selection by diverse and rapidly evolving pathogens has driven MHC class I and KIR to become diverse and fast evolving gene families that control killer lymphocytes in innate and adaptive immunity. Because human and mouse MHC class I genes are so different and KIR genes are primate-specific, these gene families will be studied in the great apes and compared to their human counterparts. This approach will achieve understanding of genetic principles underlying diversification and co-evolution of these gene families, and appreciation of their consequences for immune response and human health. Four specific aims comprise an integrated and unique attack founded on immunogenetic expertise and built upon substantial new information acquired during the last five years. Aims 1 and 2 will study a newly discovered MHC class I gene that provides an ideal system for studying the birth, differentiation and death of MHC class I genes. Aim 1 will test the hypothesis that this AL gene has evolved along very different evolutionary trajectories to become a highly polymorphic 'classical' class I gene in orangutan, a non- polymorphic 'non-classical' class I gene in chimpanzee, and a defunct relic gene in human. In chimpanzee the AL protein has properties suggestive of functions in both adaptive and innate immunity. The investigation of Aim 2 will examine functional properties of AL and search for cell-surface receptors that bind this ligand. It will reveal functions that the human immune system has lost, either through drift or selection. Aim 3 focuses on B 7301, an exceptionally divergent human HLA-B allele that retains ancestral (ape-like) features and has resisted recombination. Three alternative models that can explain these unusual properties: selection, chromosomal rearrangement, and population isolation, will be tested and distinguished by genomic analysis of the regions flanking B 7301. The orangutan is the species closest to humans in which the content of expressed MHC class I genes is markedly different. Aim 4 focuses on orangutan MHC class I and KIR and their interaction in NK cell regulation. This will test the hypothesis that orangutan is a species in which the HLA-C-mediated regulation of NK cells, which is dominant in humans, is at an intermediate stage of development.